Photomask and pattern forming method employing the same

ABSTRACT

A semitransparent phase shifting mask has, in the periphery of a pattern element area, a light shielding portion which is formed by a semitransparent phase shifting portion and a transparent portion with the optimal size combination. A pattern is formed employing the semitransparent phase shifting mask.

This is a continuation application of U.S. Ser. No. 09/893,532, filedJun. 29, 2001, now U.S. Pat. No. 6,383,718 now allowed; which is acontinuation application of U.S. Ser. No. 09/577,367, filed May 23,2000, now U.S. Pat. No. 6,258,513; which is a continuation applicationof U.S. Ser. No. 09/359,732, filed Jul. 23, 1999, now U.S. Pat. No.6,087,074; which is a continuation application of U.S. Ser. No.09/188,368, filed Nov. 10, 1998, now U.S. Pat. No. 6,013,398; which is acontinuation application of U.S. Ser. No. 08/904,754, filed Aug. 1,1997, now U.S. Pat. No. 5,851,703; which is a continuation applicationof U.S. Ser. No. 08/699,732, filed Aug. 20, 1996, now U.S. Pat. No.5,656,400; which is a continuation application of U.S. Ser. No.08/418,402, filed Apr. 7, 1995, now U.S. Pat. No. 5,578,421; which is adivisional application of U.S. Ser. No. 08/162,319, filed Dec. 7, 1993,now U.S. Pat. No. 5,429,896.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a photomask which is used tomanufacture a semiconductor device and the like, and more particularlyto a photomask which has been subjected to a processing of shifting aphase of exposure light beams and a pattern forming method employing thesame.

Along with an increase of the integration scale for semiconductordevices, sizes of patterns for forming constituent elements of thedevices become fine, and size equal to or smaller than the criticalresolution of a projection aligner are required. As a method offulfilling such a request, in JP-B-62-50811 published on Oct. 27, 1987,and corresponding to JP-A-57-62052 (laid open on Apr. 14, 1982) forexample, a photomask is employed in which a transparent film forshifting a phase of exposure light beams is provided on one oftransparent portions on the opposite sides sandwiching an opaqueportion, and thus the resolution of a pattern is exceptionally improved.

In the above-mentioned prior art, a phase shifter needs to be arrangedin one of the transparent portions adjacent to each other, and for thearrangement of the phase shifter in the complicated element pattern,high trial and error is necessarily required. Thus, there is requiredconsiderable labor. In addition, since the number of processes ofmanufacturing a photomask is doubled as compared with the prior art, thereduction in yield and the increase in cost become problems.

Those problems can be settled by employing a semitransparent phaseshifting mask in which a semitransparent portion and a transparentportion are provided, and a little quantity of light beams passedthrough the semitransparent portion is phase-inverted with respect tolight beams having passed through the transparent portion. With respectto this point, the description will hereinbelow be given with referenceto the accompanying drawings.

FIG. 1A is a cross sectional view showing a structure of an example of asemitransparent phase shifting mask. In the figure, reference numeral 1designates a transparent substrate, and a reference numeral 2 designatesa semitransparent film. A thickness of the semitransparent film 2 isadjusted such that the light beams having passed through the transparentportion are phase-inverted with respect to the light beams having passedthrough a semitransparent portion 4. The semitransparent film 2 has atransmittance such that a light beam having passed through thetransparent substrate 1 and the semitransparent film 2 has an intensityhigh enough to cause an interference with a light beam having passedthrough the transparent substrate 1. The transparent film used in thisspecification means a film having the above-mentioned transmittance. Thelight intensity distribution of the projected light beams on a waferbecomes, as shown in FIG. 1B, a sharp light intensity distribution. Thereason such a sharp light intensity distribution is obtained is thatsince the light beams having passed through the transparent portion arephase-inverted with respect to the light beams having passed through thesemitransparent portion, the former and the latter cancel each other ina boundary portion of the pattern so that the light intensity becomesapproximately zero. In addition, since the intensity of the light beamshaving passed through the semitransparent portion is adjusted to theintensity equal to or lower than the sensitivity of a photoresist, theintensity of the light beams having passed through the semitransparentportion is not an obstacle to the formation of the pattern. That is, inthis method, since the phase inversion effect between the pattern to betransferred and the semitransparent portion therearound is utilized,there is no need to take, as in the normal phase shifting mask, thearrangement of the phase shifter into consideration. In addition, in theprior art phase shift mask, the two lithography processes are requiredfor the formation of the mask. However, in this method, one lithographyprocess has only to be performed. Thus, it is possible to form the maskvery simply.

In this method, the light beams the intensity of which is equal to orlower than the sensitivity of a photoresist, to which the pattern of themask is to be transferred are made to pass through the semitransparentfilm so that the light beams which have passed through thesemitransparent film are phase-inverted with respect to the light beamswhich have passed through the transparent portion, and thus, thecontrast of the pattern is improved. As a result, it is possible toimprove the resolution of an aligner for transferring the mask pattern.The basic principle of the semitransparent phase shifting mask isdescribed in D. C. Flanders et al.: “Spatial period division—A newtechnique for exposing submicrometer—linewidth periodic andquasi-periodic patterns” J. Vac. Sci. Technol., 16(6), November/Decemberpp 1949 to 1952 (1979), U. S. Pat. Nos. 4,360,586 and 4,890,309 andJP-A-4-136854 (laid open on May 11, 1992).

In the lithography process in which the above-mentioned semitransparentphase shifting mask is employed, in the normal exposed area, goodpattern formation can be performed. However, it has been made clear bythe investigations made by the present inventors that since in theactual exposure of the wafer, the mask pattern is repeatedly transferredby the step and repeat, the light beams which have leaked from thesemitransparent area, which is located outside the periphery of theactual pattern element corresponding to an active region of a substrate,leak out to the adjacent exposed area, and thus this is an obstacle togood pattern formation.

It is therefore an object of the present invention to provide aphotomask by which a good pattern can be obtained even in the case of anexposure, in which a mask pattern is repeatedly transferred by the stepand repeat exposure, and a pattern forming method employing the same.

According to one aspect of the present invention, the above-mentionedobject can be attained by effectively making a light-shielding or opaquearea of a semitransparent phase shift mask which is located outside theperiphery of a pattern element formation area of the semitransparentphase shifting mask.

These and other objects and many of the attendant advantages of theinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are respectively a cross-sectional view showing astructure of a semitransparent phase shifting mask, and a view showingthe light intensity distribution of projected light beams on a waferwhen using the mask shown in FIG. 1A.

FIGS. 2A and 2B are respectively a plan view and a cross sectional vieweach showing a structure of a photomask according to the presentinvention.

FIG. 3A is a plan view showing a structure of a light shielding portionof the photomask according to the present invention.

FIG. 3B is a graphical representation showing the relationship betweenthe size of a transparent pattern of the photomask according to thepresent invention and the intensity of projected exposure light beams.

FIG. 4 is a plan view showing a structure of a mask for forming contactholes of a 64 Mbits-DRAM according to the present invention.

FIGS. 5A and 5B are respectively a plan view showing a structure of awindow pattern portion for aligning the position of the mask accordingto the present invention, and a view showing the light intensitydistribution of the projected light beams on the wafer when using themask shown in FIG. 5A.

FIGS. 6A through 6D are cross sectional views showing steps of a processof manufacturing a semiconductor device according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A first embodiment of the present invention will hereinafter bedescribed in detail. FIGS. 2A and 2B are respectively a plan view and across sectional view each showing the appearance of a photomask employedin the present embodiment. In those figures, reference numeral 1designates a transparent substrate, and reference numeral 5 designatesan element pattern portion in which both a semitransparent phaseshifting portion and a transparent portion are arranged. Moreover,reference numeral 6 designates a portion acting, on a wafer, as a lightshielding portion in which semitransparent phase shifting patterns arearranged at a pitch equal to or smaller than the resolution. Referencenumeral 7 designates a masking blade for shielding, on the aligner side,the exposure light beams. Since the masking blade 7 is poor in thepositional accuracy, it is positioned so as to shield the light beamspassing through the portion which is located outside the intermediateposition of the width of the area 6 acting as the light shieldingportion. The details of the area 6 acting as the light shielding portionwill hereinbelow be described with reference to FIGS. 3A and 3B. FIG. 3Ais a plan view showing a structure of a pattern. In this connection,each transparent pattern portion 10 is formed within a semitransparentphase shifting portion 9. An arrangement pitch 11 of the transparentpatterns 10 is determined depending on the resolution characteristics ofthe projection optical system employed. The arrangement pitch P isexpressed by the following expression:

P=α·λ/NA,

where NA represents a numerical aperture of a projection lens, λrepresents a wavelength of the exposure light beams, and α represents acoefficient. In this connection, on the basis of the experiments made bythe present inventors, it is desirable that the coefficient α is set toa value equal to or smaller than 0.8. However, the optimal value of α isnot limited thereto or thereby because the optimal value of α depends onthe characteristics of the illuminating system, the patternconfiguration and the like. A width 12 of the transparent pattern 10influences largely the formation of a dark portion. When both asemitransparent phase shifting pattern and a transparent pattern arearranged with the same size and at a pitch equal to or lower than thecritical resolution, a pattern image can be erased. But, in this case,the resulting uniform light intensity does not become zero. This reasonis that since there is a difference between the quantity of light beamshaving passed through the semitransparent phase shifting portion andthat of light beams having passed through the transparent portion, thefunction of cancelling those light beams each other due to the phaseinversion effect is not efficiently performed. Then, when the ratio ofthe area of the semitransparent phase shifting portion to that of thetransparent portion is adjusted in accordance with a set transmittanceof the semitransparent phase shifting portion, it is made clear that thelight intensity can be zero.

FIG. 3B shows the intensity of the projected light beams which isobtained on the wafer when changing the width 12 of the transparentpattern 10. Then, the intensity of the projected light beams shows theintensity of the light beams which have passed through the pattern alonga line A-B of FIG. 3A. The pitch of the transparent patterns 10 wasdetermined to be 0.4 μm by using α=0.1 in the expression of thearrangement pitch. With respect to the three kinds of transmittance 9%,16% and 25% of the semitransparent phase shifting portion, the change inthe intensity of the projected light beams were examined by changing thesize of the transparent pattern 10. The axis of abscissa of the graphrepresents the size of the transparent pattern 10. From the graph ofFIG. 3B, it can be seen that a minimum value is present in the intensityof the projected light beams depending on the size of the transparentpattern, and this local minimum value is variable depending on thetransmittance of the semitransparent phase shifting portion. That is, itcan be seen that in accordance with the transmittance of thesemitransparent phase shifting portion 9, an optimal transparent patternsize can be found. Denoting the size ratio of size 12 of the transparentpattern to the size 13 of the semitransparent phase shifting portion 13by α, an optimal value thereof for the formation of the dark portionwill be expressed by the following expression:

α=β·T,

where T represents a transmittance of the semitransparent phase shiftingportion, and β represents a coefficient. The allowable intensity of theprojected light beams is variable depending on intended purposes. In thecase of preventing exposure of a photoresist due to a double exposure,the allowable intensity of the projected light beams may be set to anintensity which is about one-half the intensity of light having passedthrough the semitransparent phase shifting portion. However, in the caseof preventing a double exposure of a dark portion with a fine patterncontaining portion, the change in the size of the fine pattern needs tobe reduced as much as possible, and thus it is desirable that theallowable intensity of the projected light beams is set to a value equalto or lower than 0.05. The value of β in this case is in the range ofabout 0.5 to about 2.0. Then, the area 6 of FIG. 2A was formed on thebasis of the optimal conditions thus obtained, and by actually using theprojection aligner, the pattern element 5 corresponding to the activeregion was exposed by the step and repeat. As a result, the good patternelement corresponding to the active region could be formed withoutoccurrence of the pattern destruction and the size shifting even in thearea in which the area 6 was double-exposed. As described above, thesemitransparent phase shifting portion and the transparent portion wereformed with the optimal size combination, whereby the effective darkportion could be formed. Incidentally, although in the presentembodiment, the example is shown in which the line transparent patternis formed in the semitransparent phase shifting area, the presentinvention is not limited thereto or thereby. That is, for example, thereis particularly no problem even in the case of an island-like patternand other patterns. In such cases, if α in an expression of α=β·T isreplaced with the area ratio of the area of the transparent pattern tothe area of the semitransparent phase shifting portion, thesubstantially same effects can be obtained. In addition, in the presentembodiment, the combination of the semitransparent phase shiftingpattern and the transparent pattern is applied to the prevention of thedouble exposure. However, the application of the dark portion of thepresent invention is not limited thereto or thereby. It is, of course,to be understood that the dark portion is applicable to the necessaryportions such as a window pattern for aligning the mask position, apattern for detecting the wafer position, and a semitransparent phaseshifting portion having a large area all of which require a darkportion. Further, the above-mentioned photomask having a light shieldingportion is useful for the pattern formation when manufacturing asemiconductor device.

Incidentally, the above-mentioned light shielding portion is applicableto the formation of a light shielding portion in a pattern elementregion of a substrate. In this case, since the ratio of thetransmittance of the transparent portion to that of the light shieldingportion can be made large, it is possible to increase the tolerance forthe variation of the quantity of light beams required for the exposure.

As for the materials used for the formation of the semitransparent phaseshifting portion, a lamination film of a semitransparent metal film(made of chromium, titanium or the like) or a silicide film (e.g., amolybdenum silicide film) and a silicon oxide film for the phase shift,or a single layer film such as a metal oxide film (e.g., a chromiumoxide film) or metal nitride film (e.g., a chromium nitride film) may beemployed. In the case where a single layer film such as a chromium oxidefilm or a chromium nitride film is employed, since the refractive indexthereof is larger than that of the silicon oxide film, the film can bethinned. As a result, since the influence of the light diffraction canbe reduced, this single layer film is suitable for the formation of afine pattern.

Embodiment 2

A second embodiment of the present invention will hereinafter bedescribed with reference to FIG. 4. FIG. 4 is a plan view showing astructure of a photomask which is used to form contact holes of a 64Mbit-dynamic random access memory (DRAM). Two DRAM element areas 5 arearranged in a transparent substrate 1. A scribing area 14 is providedbetween the two pattern element areas 5. In addition, in a peripheralscribing area 15 on two sides perpendicular to each other, a pattern formeasuring the accuracy of the mask alignment, a target pattern for themask alignment, and the like are arranged, which becomes necessary forthe process of manufacturing a device.

In the two sides opposite to the other sides of the scribing area, apattern configuration 6′ of light shielding portion 6 of the presentinvention is arranged. The step-and-repeat process in the projectionaligner is performed at a pitch 16 in the transverse direction and at apitch 17 in the longitudinal direction. The peripheral portion which islocated outside a dotted line 18 as the setting center is mechanicallyshielded from the light beams by a mechanical light shielding plate ofthe aligner. In this connection, the dotted line 18 is set at a distanceequal to or longer than the positional accuracy of the mechanical lightshielding plate from the scribing area such that the mechanical lightshielding plate is not shifted to the scribing area by mistake. Inaddition, the width of the pattern configuration 6′ is set to a valueequal to or larger than the positional accuracy of the mechanical lightshielding plate, and the dotted line 18 is arranged in about the centralportion of the pattern configuration 6′. Further, at least three of thefour corner portions have pattern configurations.

As a result of using this photomask in order to manufacture the 64Mbit-DRAM, the double exposure in the periphery of the chip can beperfectly prevented, and thus a good device can be manufactured. Inaddition, in the case where the pattern element area 5 is formed by onechip, or the photomask having the pattern configuration 6′ is applied todevices other than DRAM, the same effects can be obtained.

Further, the description will hereinbelow be given with respect to anexample in which the dark portion of the present invention is arrangedin the periphery of a window pattern which is used to align the maskposition with reference to FIGS. 5A and 5B. FIG. 5A is a plan viewshowing a structure of the window pattern portion which is used to alignthe mask position. FIG. 5B shows the distribution of the light intensityon the wafer corresponding to the mask position. As shown in FIG. 5A, atransparent portion 10 which has the size fulfilling the conditions forforming the dark portion of FIG. 3B is formed around a window pattern19. It can be seen that in the distribution of the light intensity onthe wafer along a line A-B of the photomask of FIG. 5A at that time, thelight intensity in the periphery of the window pattern is, as shown inFIG. 5B, zero, and thus signals representing the window pattern areobtained in the high signal-to-noise (S/N) ratio and the judgement ofthe position is performed with accuracy. In such a way, the dark portionof the present invention is applicable to a pattern utilizing lightintensity signals each having a high S/N ratio from a mask pattern andother patterns requiring the light shielding portion as well as to thelight shielding in the periphery of a device chip.

Embodiment 3

Hereinbelow, an example will be shown in which a semiconductor device ismanufactured according to the present invention. FIGS. 6A through 6D arecross sectional views showing steps of a process of manufacturing asemiconductor device. By using the conventional method, a P type welllayer 21, a P type layer 22, a field oxide film 23, a polycrystallineSi/SiO₂ gate 24, a high impurity concentration P type diffusion layer25, a high impurity concentration N type diffusion layer 26, and thelike are formed in an N⁻ type Si substrate 20. Next, by using theconventional method, an insulating film 27 made of phosphor silicateglass (PSG) is deposited thereon. Next, a photoresist 28 is appliedthereto, and then a hole pattern 29 is formed by using thesemitransparent phase shifting mask of the present invention (refer toFIG. 6B). Next, an insulating film 27 is selectively etched by the dryetching technique with the resultant photoresist as an etching mask,thereby to form contact holes 30 (refer to FIG. 6C). Next, by using theconventional method, a W/TiN electrode wiring 31 is formed, and then aninterlayer insulating film 32 is formed. Next, a photoresist is appliedthereto, and then by using the conventional method, a hole pattern 33 isformed using the semitransparent phase shifting mask of the presentinvention. Then, a W plug is plugged in the hole pattern 33 to connect asecond level Al wiring 34 thereto (refer to FIG. 6D). In the followingpassivation process, the conventional method is employed. Incidentally,in the present embodiment, only the main manufacturing processes havebeen described. In this connection, the same processes as those of theconventional method are employed except that the semitransparent phaseshifting mask of the present invention is used in the lithographyprocess of forming the contact hole. By the above-mentioned process,CMOS LSI can be manufactured at a high yield.

As set forth hereinabove, according to the present invention, it ispossible to prevent the double exposure on the wafer, and a pattern ofconstituent elements as desired can be formed. By forming thesemitransparent phase shifting portion and the transparent portion withthe optimal size combination, even if a light-shielding film is notnewly formed, the effective dark portion can be formed. In addition,without increasing in the number of processes of forming the mask, thesemitransparent phase shifting mask which is useful in practical use canbe produced. Further, as a result of manufacturing the semiconductordevice by using the photomask of the present invention, it is possibleto form the pattern in which the effects inherent in the semitransparentphase shifting mask are sufficiently utilized, without any problem inthe double exposure portion, and also it is possible to realize thereduction of the device area.

It is further understood by those skilled in the art that the foregoingdescription is a preferred embodiment of the disclosed device and thatvarious changes and modifications may be made in the invention withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising conducting pattern-transference in a step-and-repeat exposurefashion by illuminating a resist film on a semiconductor substrate withlight through a mask including (a) an element pattern area having atransparent portion and a semitransparent phase shifting portion inwhich a light beam having passed through said semitransparent phaseshifting portion has a phase substantially 180 degrees out-of-phase withrespect to a phase of a light beam having passed through saidtransparent portion and (b) a peripheral edge area at an outer peripheryof said element pattern area, in such a manner that the illuminatedresist film has first and second areas serving as light-shielding areas,said first and second areas of the illuminated resist film correspondingto said semitransparent phase shifting portion and said peripheral edgearea of said mask, respectively, wherein, in said pattern transference:light intensity on said second area of said resist film is lower thanthat on said first area of said resist film; and a masking blade isdisposed so that the masking blade overlaps said peripheral edge area ofsaid mask in such a manner that exposure light to said peripheral edgearea of said mask is partly shielded by said masking blade.
 2. A methodaccording to claim 1, wherein said semitransparent phase shiftingportion of the mask has a transmittance not higher than 25%.
 3. A methodaccording to claim 1, wherein said second area of the resist filmcorresponding to said peripheral edge area of the mask isdouble-exposed.
 4. A method according to claim 1, wherein saidperipheral edge area of the mask has a repetition of a transparentsegment and a semitransparent phase-shifting segment, with a repetitionpitch P defined as P=α·λ/NA, where NA represents a numerical aperture ofa projection lens, λ represents a wavelength of exposure light, andα≦0.8.
 5. A method according to claim 4, wherein a ratio α of an area ofsaid transparent segment to an area of said semitransparent segment insaid peripheral edge area of the mask is α=β·{square root over (T)},where T represents a transmittance of said transparent segment, and0.5≦β≦2.0.
 6. A method of manufacturing a semiconductor devicecomprising the steps of: preparing a photomask including an element areaand a substantially dark area at an outer periphery of said elementarea, said element area having a transparent area portion and asemitransparent area portion, a light beam having passed through saidsemitransparent area portion has a phase substantially 180 degreesout-of-phase with respect to a phase of a light beam having passedthrough said transparent area portion; mounting said photomask on anexposure apparatus provided with said masking blade; and conductingstep-and-repeat exposure, by use of said photomask, of a resist film ona semiconductor substrate in such a manner that said masking blade lightshields a part of said dark area of the photomask.
 7. A method accordingto claim 6, wherein said dark area of the photomask includes at least anarea corresponding to an area on said resist film to be double-exposed.8. A method of manufacturing a semiconductor device comprisingconducting pattern transference on a semiconductor wafer by use of aphotomask with double-exposure being prevented, wherein said photomaskincludes an element pattern area and a light shielding area, saidelement pattern area having semitransparent phase-shifting patterns anda transparent pattern, said light shielding area having a projectedlight intensity lower than that of a semitransparent phase-shiftingpattern arranged to surround said element pattern area.